linux-kernel - Re: [PATCH] drm/ssd130x: Fix an oops when attempting to update a disabled plane

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Message-ID: <87h6q2kh6s.fsf@minerva.mail-host-address-is-not-set>
Date:   Mon, 17 Jul 2023 11:42:19 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Thomas Zimmermann <tzimmermann@...e.de>
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/ssd130x: Fix an oops when attempting to update a
 disabled plane

Geert Uytterhoeven <geert@...ux-m68k.org> writes:

Hello Geert and Thomas,

> Hi Thomas,
>
> On Mon, Jul 17, 2023 at 10:48 AM Thomas Zimmermann <tzimmermann@...e.de> wrote:

[...]

>>
>> After some discussion on IRC, I'd suggest to allocate the buffer
>> somewhere within probe. So it will always be there when the plane code runs.
>>
>> A full fix would be to allocate the buffer memory as part of the plane
>> state and/or the plane's atomic_check. That's a bit more complicated if
>> you want to shared the buffer memory across plane updates.
>
> Note that actually two buffers are involved: data_array (monochrome,
> needed for each update), and buffer (R8, only needed when converting
> from XR24 to R1).
>
> For the former, I agree, as it's always needed.
> For the latter, I'm afraid it would set a bad example: while allocating
> a possibly-unused buffer doesn't hurt for small displays, it would
> mean wasting 1 MiB in e.g. the repaper driver (once it has gained
> support for R1 ;^).
>

Maybe another option is to allocate on the struct drm_mode_config_funcs
.fb_create callback? That way, we can get the mode_cmd->pixel_format and
determine if only "data_array" buffer must be allocated or also "buffer".

> Gr{oetje,eeting}s,
>
>                         Geert
>
> -- 
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds
>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat